Evaporated gas supply method

Abstract: In an evaporated gas supply method in which the liquid seal obtained by expansion of the evaporated gas can be effectively avoided, which comprises the step of cooling down the evaporated gas (2) in a cylinder (1), whereby the enthalpy of the evaporated gas (2) filled in the said cylinder (1) is increased over the enthalpy of the secondary pressure on the saturated vapor line in a pressure-enthalpy diagram of the same evaporated gas (2), and the step of adiabatically expanding the evaporated gas (2) filled in the cylinder (1) so that its pressure is reduced, and supplying the evaporated gas having the reduced pressure to said consuming installation (3).

Description: Detailed Description of the Invention Technical Field The present invention related to an evaporated gas supply method in which an evaporated gas having a primary pressure filled in a cylinder is reduced in pressure to a secondary pressure through adiabatic expansion, and the evaporated gas having the secondary pressure is supplied to a predetermined consuming installation. Prior Art JP-A-1-266 400 discloses a method and device for preventing the creeping phenomenon of doping gas in a pressure regulator in which a relatively high pressure gas stored in a gas cylinder is adiabatically expanded via an expansion valve to a consuming installation at a lower pressure. In order to prevent the creeping phenomenon due to low vapor pressure impurities, a cooling mechanism is added to the gas cylinder so that the temperature of the supply gas at the exit of said gas cylinder becomes equal to or lower than the gas temperature at the gas exit of the pressure regulator. US-A-4 878 510 discloses a method which avoids the formation of droplets by condensation of vapors during the expansion of a highly compressed gas through a critical orifice. The pressure drops is distributed over a sufficient number of critical orifices so as to limit the temperature drop insufficient to initiate droplet formation. For the purpose of realizing a long-term supply of an evaporated gas in a case where the evaporated gas is supplied to a semiconductor manufacturing factory, such a conventional evaporated gas supply system has been used that the said evaporated gas is filled at a high-pressure condition (for example, 51 bars.abs [52 Kg/cm 2 .abs]) in a cabinet cylinder, and it will be supplied after its pressure is reduced by an expansion valve. Referring to Fig.10 and Fig. 11, the evaporated gas supply system of the prior art will be described. Fig.10 is a view showing the outline of a conventional evaporation and supply apparatus and Fig.11 is a graph showing a change of pressure in an expansion valve. In a cabinet cylinder 1, an evaporated gas 2 is filled at a high-pressure (primary pressure) condition. A pipe 4 is laid from this cabinet cylinder 1 to a consuming installation 3 such as a semiconductor manufacturing factory, and an expansion valve 5 is attached on the way thereof. By means of this expansion valve 5, the evaporated gas 2 in the cabinet cylinder 1 is reduced in pressure so as to provide the evaporated gas 2 having a low pressure (secondary pressure) usable in the consuming installation 3. Problems Sought for Solution by the invention. According to the evaporated gas supply method of the prior art, the evaporated gas 2 is being preserved in the cabinet cylinder 1 for a long period of time, and therefore, the evaporated gas 2 to be supplied gets in a saturated state or a state near thereto. If the evaporated gas 2 is supplied from such state by way of the expansion valve 5, a liquid seal will be caused to take place. For instance, in a case where 100% N 2 O having a primary pressure of 49 bars (50 kg/cm 2 ) is led to the expansion valve 5 so that its pressure is reduced to a secondary pressure of 4,9 bars (5 kg/cm 2 ), it will become N 2 O comprising 97% of gas and 3% of liquid. As a result, a liquid seal takes place, resulting in such a problem that the same evaporated gas 2 can not be supplied at a predetermined flow rate to the consuming installation 3. It is an object of the present invention to provide an evaporated gas supply method in which the liquid seal of an evaporated gas can be prevented from taking place. The process according to the invention relates to an evaporated gas supply method in which an evaporated gas having a primary pressure filled in a cylinder is reduced in pressure to a secondary pressure through adiabatic expansion , and the evaporated gas having the secondary pressure is supplied to a predetermined consuming installation , said method further comprising : a first step of cooling the evaporated gas in the cylinder to a temperature whereby the enthalpy of the evaporated gas filled in said cylinder is increased over the enthalpy at the secondary pressure of the gas on the saturated vapor line in a pressure-enthalpy diagram of the same evaporated gas such that liquefaction of the evaporated gas can be prevented from taking place ;and a second step of adiabatically expanding the evaporated gas filled in the cylinder so that it's pressure is reduced to the secondary pressure ; and a third step of supplying the evaporated gas having the reduced secondary pressure to said consuming installation , and wherein the state of enthalpy of the gas in the cylinder is monitored by detecting the temperature and/or pressure of the gas in said cylinder . Function Since the present invention is constructed as mentioned above, an evaporated gas filled in a cylinder is cooled down before it is adiabatically expanded so that its enthalpy becomes at least an enthalpy at the secondary pressure on the saturated vapor lane. If this evaporated is adiabatically expanded, its pressure will be reduced to a secondary pressure, but no liquefaction is caused to take place at the secondary pressure for supply to the consuming installation because the adiabatically expanded evaporated gas has already obtained the enthalpy at the secondary pressure on the saturated vapor line. Thus, its liquid seal can be prevented from taking place. Embodiment Referring to the accompanying drawings, the evaporated gas supply method according to one preferred embodiment of the present invention will be described. The same reference numerals are used for the same elements and the duplicate explanations will be omitted. The principle of the present invention will be first described in accordance with Figs 2 to 8. Each of Figs 2 to 8 is a pressure-enthalpy diagram of N 2 O showing a change of state where its pressure is reduced from a primary pressure P1 to a secondary pressure P2=3,9 bars.abs (P2=4 kg/cm 2 .abs). As to the primary pressures P1, Fig.2 shows P1=80,4 bars.abs (P1=82 kg/cm 2 .abs), Fig.3: P1=72,5 bars.abs (P1=74 kg/cm2.abs), Fig.4: P1= 62,7 bars.abs (P1=64 kg/cm 2 .abs), Fig.5: P1= 51 bars.abs (P1=52 kg/cm 2 .abs), Fig.6: P1=42,1 bars.abs (P1=P1=43 kg/cm 2 .abs), Fig.7: P1=33,3 bars.abs (P1=34 kg/cm 2 .abs) and Fig.8: P1= 24,5 bars.abs (P1=25 kg/cm 2 .abs), respectively. In Fig.2, a change of state is shown when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 40°C is reduced in pressure from P1=80,4 bars.abs (P1=82 kg/cm 2 .abs) (Point A) to P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point B) by an expansion valve 5. In this case, the temperature of N 2 O caused by its adiabatic expansion in the pressure reduction is lowered to about -63°C and about 18% of the gas is liquefied. Fig.3 shows a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 36,5°C, this is the critical temperature of N 2 O, is reduced in pressure from P1=72,5 bars.abs (P1=74 kg/cm 2 .abs) (Point A) to P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point B) by an extension valve 5. In this case, the temperature of N 2 O caused by its adiabatic expansion in the pressure reduction is lowered to about -63°C and about 22% of the gas is liquefied. Fig.4 shows a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 30°C is reduced in pressure from P1= 62,7 bars.abs (P1=64 kg/cm 2 .abs) (Point A) to P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point B) by an expansion valve 5. In this case, the temperature of N 2 O caused by its adiabatic expansion in the pressure reduction is lowered to about -63°C and about 10% of the gas is liquefied. In Fig.5, a change of state is shown when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 20°C is reduced in pressure from P1=51 bars.abs (P1=52 kg/cm 2 .abs) (Point A) to P2=3,9 bars .abs (P2=4 kg/cm 2 .abs) (Point B) by an expansion valve 5. In this case, the temperature of N 2 O caused by its adiabatic expansion in the pressure reduction is lowered to about -63°C and about 3% of the gas is liquefied. Fig.6 shows a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 10°C is reduced in pressure from P1=42,1 bars.abs (P1=43 kg/cm 2 .abs) (Point A) to P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point B) by an expansion valve 5. In this case, the temperature of N 2 O caused by its adiabatic expansion in the pressure reduction is lowered to about -63°C, but the gas is not liquefied even by the adiabatic expansion because Point B is on the saturated vapor line (a critical line of saturation). For facilitating the expansion, in addition, the enthalpy on the critical line of saturation at P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) will be hereinafter defined as "the saturation critical enthalpy (=h 2sc )". Fig.7 shows a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 0°C is reduced in pressure from P1=33,3 bars.abs (P1=34 kg/cm 2 .abs) (Point A) to P2=3,9bars.abs (P2=4 kg/cm 2 .abs) (Point B) by an expansion valve 5. In this case, the temperature of N 2 O caused by its adiabatic expansion in the pressure reduction is lowered to about -60°C, but the gas is not liquefied even by the adiabatic expansion because Point B is right of the critical line of saturation, namely it is in a state of superheated vapor. In Fig.8, furthermore, a change of state is shown when N 2 O filled in a cabinet cylinder 1 whose internal temperature is -10°C is reduced in pressure from P1=25 kg/cm 2 .abs (Point A) to P2=3,9 bars.abs (P2=4/cm 2 .abs) (Point B) by an expansion valve 5. In this case, the temperature of N 2 O caused by its adiabatic expansion in the pressure reduction is lowered to about -57°C, but no liquefaction of the gas is absolutely caused to take place in the expansion valve 5 because both the adiabatic expansion course and Point B are right of the critical line of saturation. In addition, P1=24,5 bars.abs (P1=25 kg/cm 2 .abs) exhibits the maximum enthalpy on the critical line of saturation, and for facilitating the explanation, this pressure will be hereinafter defined as "the maximum enthalpy pressure (=P hmax )". From these inventor's data obtained as mentioned above, the inventor has discovered: 1 ○ If the state of the evaporated gas 2 fed from the cabinet cylinderl to the expansion valve 5 is kept in anyone of the states shown in Fig.6, Fig.7 and Fig.8, the liquefaction of the evaporated gas 2 in the expansion valve 5 can be prevented from taking place; and 2 ○ In order that the state of the evaporated gas 2 is transferred from the state shown in Fig.2, Fig.3, Fig.4 or Fig.5 to the state shown in Fig.6, Fig.7 or Fig.8, this transfer can be satisfactorily carried out by cooling down the evaporated gas 2. The discovery 1 ○ is based on the adiabatic expansion courses shown in Figs.2 to 8 and the discovery 2 ○ is based on the positional relations of the isotherms depicted in Figs.2 to 8. Referring to Fig.1, in the next place, the evaporated gas supply system which the inventor has invented will be described. Fig.1 is a schematic view showing the evaporated gas supply system according to one preferred embodiment of the present invention. The differences between the evaporated gas supply system according to the present invention and that of the prior art are the addition of a cooling means 10 to the cabinet cylinder 1 and the provision of a state change monitoring means which can detect a physical quantity (such as pressure or temperature) capable of specifying the state of the evaporated gas filled in the cabinet cylinder. In the cabinet cylinder 1, an evaporated gas 2 is filled in a high-pressure (primary pressure) condition, as mentioned above. So, a portion of the evaporated gas 2 gets liquefied. From this cabinet cylinder 1, a pipe 4 is laid to a consuming installation 3, for instance a semiconductor manufacturing factory, and an expansion valve 5 is attached on the way of said pipe 4. The pipe 4 is covered with a heat insulating material for insulation of heat from the out side. By means of this expansion valve 5, the evaporated gas 2 in the cabinet cylinder 1 is reduced in pressure so as to provide the evaporated gas 2 having a low pressure (secondary pressure) usable in the consuming installation 3. The cabinet cylinder 1 has the cooling means 10 and the state change monitoring means 11 installed thereto. An air cooling system of blowing air into the cabinet cylinder 1 or a liquid cooling system of cooling the outer periphery of the cabinet cylinder 1 with a liquid is applicable as the cooling means 10. And, a thermometer, pressure gage, hygrometer or thermostat is usable as the state change monitoring means 11. The evaporated gas supply method according to the present invention is effective in a case where (a) the primary pressure of an evaporated gas when filled in a cabinet cylinder 1 exceeds P hmax and (b) the enthalpy thereof at the primary pressure when filled in a cabinet cylinder 1 is less than h 2sc (namely, in a case where an evaporated gas having a primary pressure is adiabatically expanded as it is a result, its liquefaction is caused to take place). At first, accordingly, it must be done to judge whether the state of the evaporated gas 2 in the cabinet cylinder 1 satisfies the above conditions (a) and (b). In a case where these conditions are satisfied, the cabinet cylinder 1 is cooled means 10 because the evaporated gas will be liquefied, if it is adiabatically expanded as it is. By this cooling of the cabinet cylinder 1, the evaporated gas 2 having the primary pressure goes down along the saturated vapor tine, the change of state of the evaporated gas 2 caused by the cooling means 10 is always monitored by means of the state change monitoring means 11, and the detection of this state will be continued until its enthalpy reaches h 2sc. As to the detecting method, it may be satisfactorily carried out to detect the enthalpy of the evaporated gas 2 directly or to detect its pressure or temperature that exhibits indirectly the state of enthalpy. After the enthalpy of the evaporated gas 2 has reached the desired state, the evaporated gas 2 in the cylinder cabinet 1 is fed to the expansion valve 5, where its pressure is reduced for delivery to the consuming installation 3. Owing to the above mentioned construction, the present invention can reliably prevent the liquefaction of an evaporated gas in an expansion valve from taking place. In particular, in a case where a device (such as a regulator) which undergoes any bad influence by an evaporated gas containing several percent of liquid is placed downstream of the expansion valve, the present invention is more effective. In the above mentioned embodiment, a case where the evaporated gas is adiabatically expanded in one stage has been described as one example. However, the present invention is also applicable to a case the adiabatic expansion of an evaporated gas must be cooled down so that the enthalpy of the evaporated gas before a final adiabatic expansion is carried out reaches h 2sc . For instance, in a case where an evaporated gas is adiabatically expanded in two stages, the enthalpy is evitably increased by Δh=21 KJ/Kg (Δh=5 kcal/kg) between the first-stage adiabatic expansion and second-stage adiabatic expansion under the condition of h 2sc =650KJ/Kg (h 2sc =155 kcal/kg) and h 1 =620 KJ/Kg (h 1 =148 kcal/kg), the aimed enthalpy should be 630 KJ/Kg (150 kcal/kg) that is obtained by substracting Δh from h 2sc . Therefore, the cabinet cylinder will be satisfactorily cooled down so that the state of the evaporated gas before it is adiabatically expanded becomes at least 630 KJ/Kg (150 kcal/kg). Fig.9 shows a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 30°C is reduced in pressure from an initial pressure P1=62,7 bars.abs (P1=64 kg/cm 2 .abs) (Point A) to a supply pressure P2= 3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point C) by two expansion valves 5. The evaporated gas is cooled down so that its first enthalpy h 1 becomes over (h 2sc -Δh), as mentioned above. Then, the evaporated gas 2 is reduced in pressure to a medium pressure PC=9,8 bars.abs (PC=10 kg/cm 2 .abs) by the first expansion valve so that its enthalpy value is increased by Δh until it is fed to the final expansion valve, and it is further reduced in pressure to a supply pressure P2=3,9 bars.abs (P2=4 kg/cm.abs) by the final expansion valve. Although the temperature of the evaporated gas is lowered to about -55°C due to its adiabatic expansion in the final pressure reduction, no liquefaction of the gas is caused to take place even by the adiabatic expansion because Point C is right of the saturated vapor line, i.e. positioned in the region of superheated vapor. In addition, the present invention is not limited to the above embodiment and various modifications can be made to the present invention. Although an expansion valve is used as the device which makes means can be used, not limited to the expansion valve, if it has a function of making the adiabatic expansion of gas. Although this preferred embodiment has been described as one example of N 2 O, the present invention is also applicable to an evaporated gas (such as HC1) in which the critical temperature is near to room temperature, the vapor pressure at room temperature is higher and the flow rate is larger. Effect of the invention The present invention is constructed as described above, and therefore, it is possible to effectively prevent the liquid deal of an evaporated gas from taking place. Brief Description of the drawings Fig. 1 is a schematic view showing the evaporated gas supply system according to one preferred embodiment of the present invention. Fig.2 is a view showing a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 40°C is reduced in pressure from P1=80,4 bars.abs (P1=82 kg/cm 2 .abs) (Point A) to P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point B) by an expansion valve 5. Fig.3 is a view showing a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 36.5°C, this is the critical temperature of N 2 O is reduced in pressure from P1=72,5 bars.abs (P1=74 kg/cm 2 .abs) (Point A) to P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point B) by an expansion valve 5. Fig.4 is a view showing a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 30°C is reduced in pressure from P1=62,7 bars.abs (P1=64 kg/cm 2 .abs) (Point A) to P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point B) by a expansion valve 5. Fig.5 is a view showing a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 20°C is reduced in pressure from P1=51 bars.abs (P1=52 kg/cm 2 .abs) (Point A) to P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point B) by a expansion valve 5. Fig.6 is a view showing a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 10°C is reduced in pressure from P1=42,1 bars.abs (P1=43 kg/cm 2 .abs) (Point A) to P2=3(P2=4 kg/cm 2 .abs) (Point B) by a expansion valve 5. Fig.7 is a view showing a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 0°C is reduced in pressure from P1= 33,3 bar.abs (P1=34 kg/cm 2 .abs) (Point A) to P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point B) by a expansion valve 5. Fig.8 is a view showing a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is -10°C is reduced in pressure from P1=24,5 bars.abs (P1=25 kg/cm 2 .abs) (Point A) to P2=3,98 bars.abs (P2=4 kg/cm 2 .abs) (Point B) by a expansion valve 5. Fig.9 is a view showing a change of state when N 2 O filled in a cabinet cylinder 1 whose internal temperature is 30°C is reduced in pressure from P1=62,7 bars.abs (P1=64 kg/cm 2 .abs) (Point A) to medium pressure PC=9,8 bars.abs (PC= 10 kg/cm 2 .abs) (Point B) to P2=3,9 bars.abs (P2=4 kg/cm 2 .abs) (Point C) by a expansion valve 5. Fig. 10 is a view showing the outline of the evaporation and supply apparatus of the prior art. Fig.11 is a view of showing the change of pressure in the expansion valve. Description of Reference Numerals 1--cabinet cylinder, 2--evaporated gas, 3--consuming installation, 4--pipe, 5--expansion valve, 10--cooling means, 11--state change monitoring means.

Claims: An evaporated gas supply method in which an evaporated gas having a primary pressure filled in a cylinder is reduced in pressure to a secondary pressure through adiabatic expansion, and the evaporated gas having the secondary pressure is supplied to a predetermined consuming installation, said method further comprising: a first step of cooling the evaporated gas in the cylinder to a temperature whereby the enthalpy of the evaporated gas filled in said cylinder is increased over the enthalpy at the secondary pressure of the gas on the saturated vapor line in a pressure-enthalpy diagram of the same evaporated gas such that liquefaction of the evaporated gas can be prevented from taking place ; and a second step of adiabatically expanding the evaporated gas filled in the cylinder so that it's pressure is reduced to the secondary pressure; and a third step of supplying the evaporated gas having the reduced secondary pressure to said consuming installation, and wherein the state of enthalpy of the gas in the cylinder is monitored by detecting the temperature and/or pressure of the gas in said cylinder. An evaporated gas supply method according to claim 1 in which the evaporated gas is N 2 O. An evaporated gas supply method according to claim 1 or 2 in which the evaporated gas in the cylinder is cooled by cooling the cylinder. An evaporated gas supply method according to one of the claim 1 to 3 in which the adiabatic expansion is achieved by the use of an expansion valve. An evaporated gas supply method according to one of the claim 1 to 4 in which in a case where the adiabatic expansion is carried out in plural stages, the enthalpy of the evaporated gas before its final adiabatic expansion is carried out is increased, at the said second step, over the enthalpy of the secondary pressure on the saturated vapor line in the pressure- enthalpy diagram of the same evaporated gas.

Inventors: YOKOGI KAZUO, YOKOGI, KAZUO, YOKOGI, KAZUO, C/O TEISAN K.K.
Applicants: AIR LIQUIDE JAPAN LTD, AIR LIQUIDE JAPAN, LTD.
Application year: 1994
Main classification: F17C7
IPC codes: F17C13, F17C7